Projection optics for microlithography

ABSTRACT

A projection optics for microlithography, which images an object field in an object plane into an image field in an image plane, where the projection optics include at least one curved mirror and including at least one refractive subunit, as well as related systems, components, methods and products prepared by such methods, are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/969,476, filed Jan. 4, 2008, which claims priority under 35 U.S.C.§119(e)(1) to U.S. provisional patent application Ser. No. 60/885,235,filed Jan. 17, 2007. U.S. application Ser. No. 11/969,476 also claimspriority under 35 U.S.C. §119 to German patent application serial number10 2007 003 307.0, filed Jan. 17, 2007. The contents of theseapplications is hereby incorporated by reference.

FIELD

The disclosure relates to a projection optics for microlithography,which images an object field in an object plane into an image field inan image plane, where the projection optics include at least one curvedmirror and including at least one refractive subunit, as well as relatedsystems, components, methods and products prepared by such methods.

BACKGROUND

Projection optics for a projection exposure system of are known. In someinstances, the components of the refractive subunits contained thereinare utilised asymmetrically and only incompletely. Also known areprojection optics for projection exposure systems, in which the imagingbeam path is guided via a beam splitter cube.

SUMMARY

In one aspect, the disclosure provides a projection optics formicrolithography which images an object field in an object plane into animage field in an image plane. The projection optics includes a curvedmirror and a refractive subunit. A reflection surface of the mirror isconfigured as a static free-form surface which cannot be described by arotationally symmetrical function.

In another aspect, the disclosure provides a microlithography projectionexposure system. The system includes an illumination optics configuredto guide illumination light toward an object field in an object plane, aprojection optics as described in the preceding paragraph.

In a further aspect, the disclosure provides a method that includesusing a microlithography projection exposure system as described in thepreceding paragraph to produce a microstructured element.

In some embodiments, the disclosure provides a projection optics for aprojection exposure system with a relatively simple design and/or thatprovides effective utilisation of the components of the at least onerefractive subunit of the projection optics over the cross section ofthese components.

In certain embodiments, a projection optics, having a reflection surfaceof at least one mirror, is configured as a static free-form surfacewhich cannot be described by a rotationally symmetrical function.

It has been recognised that the use of at least one static free-formsurface in the projection optics markedly can increase the degrees offreedom during guidance of the imaging light through the projectionoptics. The use of at least one free-form surface can allow an imagingbeam path in the at least one refractive subunit of the projectionoptics that utilises the components contained therein more effectivelythan in the prior art. The use of a smaller number of refractivecomponents accordingly can allow a predetermined image-side numericalaperture in the projection optics according to the disclosure.Conversely, a predetermined number of refractive components can allow alarger image-side numerical aperture to be produced than in the priorart. This can lead to the possibility of a projection optics havinghigher structural resolution. The free-form surface cannot be describedby a function which is rotationally symmetrical about a marked axisnormal to a surface portion of the optical surface. The free-formsurface therefore cannot, in particular, be described by a conicsection-aspherical equation. Although aspheres of this conic type differfrom a spherical symmetry, they can be described by a rotationallysymmetrical function, i.e. by a function which is dependent merely onone parameter, i.e. on the distance from an optical axis, whereas thefree-form surfaces have at least two mutually independent parameters todescribe the surface. Conic section-aspherical surfaces are thereforenot free-form surfaces according to the disclosure. The shape of theborder of the optically active surface is generally not significant inthis regard. Optically active surfaces which are non-rotationallysymmetrically bordered are known. Nevertheless, optically activesurfaces of this type can be described by a rotationally symmetricalfunction, a non-rotationally symmetrically bordered detail of thisoptical surface being used. The term “static free-form surface” refersto a free-form surface, the shape of which is not actively changedduring use of the projection optics for projection. A static free-formsurface can also be displaced as a whole for the purposes of adjustment.The free-form surface can, in particular, be designed starting from aplanar reference surface or basic shape, from a concave referencesurface or from a convex reference. In particular, use may be made of atleast one free-form surface designed starting from a curved referencesurface. In this case, use is desirably made of a reference surfacehaving a vertex curvature which is constant over the entire referencesurface. A conic section-asphere can also be used as the referencesurface. The at least one free-form surface differs from a rotationallysymmetrical surface, which is optimally adapted to the free-form surfaceand does not have to correspond to a design reference surface, at mostby an amount corresponding, in particular, at least to the amount of awavelength of the imaging light. This difference from, in particular, atleast the amount of a wavelength of the imaging light is in practicealways much greater than production tolerances in the manufacture ofoptical components for microlithography, which are in absolute termstypically 0.1 nm and in relative terms typically 1/50 or 1/100 of thewavelength of the illumination light used. If DUV (deep ultraviolet)imaging light is used, this difference is therefore typically more than100 nm, in particular even more than 500 nm or even more than 1 μm. Evenmuch greater differences between the free-form surface according to thedisclosure and an optimally adapted rotationally symmetrical surface arepossible. A free-form surface according to the disclosure may beprovided by a biconical surface, i.e. an optical surface having twodifferent basic curvatures and two different conical constants in twodirections perpendicular to each other, by a toric surface or by ananamorphous and at the same time, in particular, aspherical surface. Acylindrical surface is therefore also a freeform surface of this type.The free-form surfaces according to the disclosure can bemirror-symmetrical to one or more planes of symmetry. The free-formsurface according to the disclosure may be a surface having an n-foldsymmetry, wherein n is an integer and greater than or equal to 1. Thefree-form surface may have no axis of symmetry whatsoever and no planeof symmetry whatsoever. The projection optics can have at least twocurved mirrors mapping the object field into the image field.

Various possibilities for describing optical surfaces, in particularanamorphous surfaces, are described for example in U.S. Pat. No.6,000,798. Analytical formulae for describing non-rotationallysymmetrical surfaces, in particular anamorphous aspherical surfaces,toric surfaces or biconical aspherical surfaces, are also described inWO 01/88597 A. Insofar as the disclosure of these documents relates tothe mathematical description of optical surfaces, these documents arehereby incorporated by reference. Certain optical design programs suchas Oslo® and Code V® also allow the description and design of opticalsystems via mathematical functions via which it is also possible todefine non-rotationally symmetrical optical surfaces. Theabove-mentioned mathematical descriptions relate to mathematicalsurfaces. An optical surface which is actually optically utilised, i.e.a physical surface of an optical element that is acted on by anillumination beam and that can be described using a mathematicaldescription of this type, generally contains merely a detail of theactual mathematical surface which is also referred to as the parentsurface. The mathematical surface therefore also extends beyond thephysical, optically active surface. Insofar as an optical system can bedescribed with the aid of a reference axis, some or all of the opticallyused surface portions can be arranged outside this reference axis insuch a way that the reference axis intersects the mathematical surfacebut not the detail of this mathematical surface that is actuallyoptically utilised.

Field planes arranged parallel to one another facilitate integration ofthe projection optics into the design environment of the projectionexposure system. This can be particularly advantageous if the projectionoptics is used in a scanning projection exposure system, as the scanningdirections can then be guided parallel to one another.

Arrangements having a small object/image offset can lead to a compactprojection optics and also facilitate optic tests in which theprojection optics is pivoted about an axis extending centrally throughthe object or image field and located perpendicularly to thecorresponding field plane, as in this case the object and image fieldsdo not migrate far during swivelling of the projection optics.

Arrangements of the optical axis relative to the object or image fieldwherein the intersection of an optical axis of an refractive subunitwith the object plane is located in the object field and in particularis centred in the object field, and wherein the intersection of theoptical axis of the refractive subunit with the image plane is locatedin the image field and in particular is centred in the image field allowoptimally symmetrical beam guidance of the illumination or imaging lightalong the optical axis of the at least one refractive subunit of theprojection optics. This can be utilised for substantially rotationallysymmetrical or even completely rotationally symmetrical illumination ofthe optical components of the refractive subunit. In the event ofresidual absorption of illumination light in these optical components,possible repercussions which corresponding heating of the absorbingoptical components has on the imaging of the projection optics are alsoeither substantially or completely rotationally symmetrical or have ann-fold symmetry and can therefore be compensated for or correctedrelatively easily. The term “n-fold symmetry” means in this connectionthat rotation through an angle of 360°/n about the optical axis causesany imaging effects present, in particular imaging errors, to merge.

A projection optics having six mirrors can allow for effective wavefront and distortion correction via the image field. Alternatively, theprojection optics can also be equipped with a different number ofmirrors, for example with two or four mirrors. These mirrors may includea free-form surface, although there may also be two or more free-formsurfaces, of which at least one free-form surface may be configured as acurved free-form surface. The same applies to a refractive subunithaving at most eight lenses. The fewer lenses the refractive subunithas, the lower the transmission losses of the refractive subunit. Themore lenses the refractive subunit has, in general, the better thepossibilities for wave front and distortion correction are in practice.The refractive subunit can have at least six lenses. A refractivesubunit having eight lenses is in this regard a good compromise in termsof the number of lenses. Depending on the imaging demands placed on theprojection optics, six or fewer lenses may also be an advantageouscompromise between imaging quality and reduction of transmission losses,material and manufacturing costs.

A projection optics having an image-side and an object-side refractiveunit of the proj ection optics, between which at least two mirrors arearranged, of which at least one has a free-form surface, may beconfigured symmetrically, and this provides advantages for production inthe case of the projection optics. Complete symmetry can be achievedonly in specific cases in which the imaging scale of the projectionoptics is 1:1. A reflective subunit having two or else having fourmirrors may, for example, be arranged between the refractive subunits.

A beam guidance in a projection optics, wherein the intersection of theoptical axis of the object-side refractive unit of the projection opticswith the object plane is located in the object field and wherein theintersection of the optical axis of the image-side refractive subunit ofthe projection optics with the image plane is located in the imagefield, and a beam guidance in a projection optics, wherein theintersection of the optical axis of the object-side refractive subunitof the projection optics with the object plane is centred in the objectfield and wherein the intersection of the optical axis of the image-siderefractive subunit of the projection optics with the image plane iscentred in the image field, have, both for the image-side and for theobject-side refractive subunit, the advantages discussed hereinbefore.

A beam guidance in a projection optics, wherein a principal ray of thecentral field point between two mirrors arranged between the tworefractive subunits extends parallel to the optical axis of the tworefractive subunits and set apart from the optical axis, allows asymmetrical configuration of a reflective subunit, including the atleast two mirrors, of the projection optics between the two refractivesubunits, and this also has advantages for production in the manufactureof the optical components of the projection optics. A beam guidance inan projection optics, wherein the optical axes of the two refractivesubunits extend parallel and set apart from each other, allows, forexample, a point-symmetrical configuration of the optical components ofthe projection optics, and this also has advantages for production.

A deformable mirror in the region of a pupil plane of the projectionoptics can compensate for drift effects occurring during operation ofthe projection optics, for example as a result of thermal influences.The reflection surface of the deformable mirror can be regarded as anon-static free-form surface.

An image field which is larger than 1 mm² can result in a highthroughput of the projection optics.

An image-side numerical aperture of at least 0.5 can allow for highresolution of the projection optics. If an immersion system is used, theimage-side numerical aperture can be even larger, for example largerthan 1.0.

A telecentric projection optics on one of the group of object and imageside can increase the flexibility of use thereof. Projection opticshaving image-side telecentry have an imaging scale which is constantover the entire focus range.

A light source for generating illuimination light having a wavelength inthe range of from 126 to 248 mm can allow for good structural resolutionof the projection exposure system. Typical UV light sources which can beused have wavelengths of 126, 157, 193 and 248 nm.

A projection optics having in the imaging beam path between the objectplane and the image plane at least one intermediate image plane canallow, due to the intermediate image plane, the imaging effects of therefractive subunit to be separated from the imaging effect of the atleast one curved mirror of the projection optics. This can simplify thedesign of the projection optics.

The advantages of a projection exposure system for microlithographyincluding a light source for illumination light, an illumination opticsfor guiding the illumination light toward an object field in an objectplane and a projection optics according to the disclosure, cancorrespond to those previously listed hereinbefore with regard to theprojection exposure system according to the disclosure.

The same applies to a manufacturing method including the steps ofproviding a reticle and a wafer, projecting a structure on the reticleonto a light-sensitive layer of the wafer with the aid of the projectionexposure system described above, and producing a microstructure on thewafer, and to the microstructured structural part manufactured thereby.

Exemplary embodiments of the disclosure will be described hereinafter ingreater detail with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of a projection exposure systemfor microlithography;

FIG. 2 shows a section, including imaging beam paths of field points setapart from one another, through an embodiment of a projection optics ofthe projection exposure system according to FIG. 1;

FIG. 3 shows a diagram showing the field profile of the wave front error(rms value) of the projection optics according to FIG. 2;

FIG. 4 shows a diagram showing the field profile of the distortion ofthe projection optics according to FIG. 2;

FIG. 5 shows a section through a non-rotationally symmetrical free-formsurface and through a rotationally symmetrical reference surface;

FIG. 6 shows a view similar to FIG. 1 of a projection exposure systemfor microlithography;

FIG. 7 shows a schematic illustration of a projection optics for theprojection exposure system according to FIG. 1; and

FIG. 8 shows a schematic illustration of a projection optics for theprojection exposure system according to FIG. 1.

DETAILED DESCRIPTION

A projection exposure system 1 for microlithography has a light source 2for illumination light. The light source 2 is a UV light sourcegenerating light having a wavelength of approx. 193 nm. Light sources 2,i.e. lasers, generating wavelengths of this type are known. Otherwavelengths, for example 157 nm or 248 nm, which are suitable forrefractive media, are also possible. FIG. 1 is a highly schematicillustration of a beam path of the illumination light 3.

An illumination optics 5 is used to guide the illumination light 3toward an object field in an object plane 4. The object field is mappedinto an image field in an image plane 8 having a predetermined scale ofdemagnification using a projection optics 6. The projection optics 6,which is shown in detail in FIG. 2, demagnifies by a factor of 4. Theimage plane 8 is arranged in the projection optics 6 parallel to theobject plane 4. A detail, coinciding with the object field, of a mask 9which is penetrated by radiation and is also referred to as a reticle,is imaged. The imaging is carried out onto the surface of a substrate 10in the form of a wafer carried by a substrate holder 11. FIG. 1 showsschematically between the reticle 9 and the projection optics 6 a beamcluster 12, entering the projection optics, of the illumination light 3and between the projection optics 6 and the substrate 10 a beam cluster13, leaving the projection optics 6, of the illumination light 3. Theimage field-side numerical aperture of the projection optics 6 accordingto FIG. 2 is 0.80. The projection optics 6 according to FIG. 2 istelecentric both on the object and on the image side.

To facilitate description of the projection exposure system 1, thedrawings show a Cartesian xyz-coordinate system producing the respectivepositional relationship of the components shown in the figure. In FIG.1, the x-direction extends into the drawing plane perpendicularlythereto, the y-direction toward the right and the z-direction downward.

The projection exposure system 1 is of the scanner type. Both thereticle 9 and the substrate 10 are scanned in the y-direction duringoperation of the projection exposure system 1.

FIG. 2 shows the optical design of the projection optics 6. Illustratedis the beam path of the three respective individual beams 14 emanatingfrom five object field points which in FIG. 2 are located one aboveanother and set apart from one another in the y-direction, the threeindividual beams 14, which pertain to one of these five object fieldpoints, each being associated with three different directions ofillumination.

Starting from the object plane 4, the individual beams 14 are firstreflected by a reflective subunit 15 having a total of six mirrors 16 to21 which are numbered in the order of the beam path and willsubsequently also be referred to, likewise in the order of the beampath, as mirrors M1, M2, M3, M4, M5 and M6. The mirrors 16, 18, 19 and21 have a concave basic shape, i.e. can be described by a concaveoptimally adapted surface. The mirrors 17 and 20 have a convex basicshape, i.e. can each be described by a convex optimally adapted surface.In the remainder of the description, mirrors of this type will bedescribed simply as concave or convex.

All six mirrors 16 to 21 of the projection optics 6 are configured asfree-form surfaces which cannot be described by a rotationallysymmetrical function. Also possible are other embodiments of theprojection optics 6, in which at least one of the mirrors 16 to 21 has afree-form reflection surface of this type. At least one reflectionsurface is in this case configured as a static free-form surface, i.e. asurface, the shape of which cannot be purposefully altered duringoperation or during downtime of the projection exposure system 1, whichsurface cannot be described by a rotationally symmetrical function.

In the exemplary embodiment shown in FIG. 2, planar pupil orintermediate image planes may generally not be allocated to theprojection optics 6. Both pupils and intermediate images are generatedin the reflective subunit 15 on pupil surfaces or intermediate imagesurfaces extending transversely to the beam path of the individual beams14 having complex topography. Surfaces of this type, which are indicatedin FIG. 2 by straight or curved lines, will be referred to hereinaftersimply as pupil or intermediate image planes.

A first pupil plane 22 is located between the first mirror 16 and thesecond mirror 17 of the projection optics 6. A first intermediate imageplane 23 of the projection optics 6 is located between the fourth mirror19 and the fifth mirror 20. The numerical aperture in the intermediateimage plane 23 is approximately 0.17. A second pupil plane 24 of theprojection optics 6 is located between the fifth mirror 20 and the sixthmirror 21.

A marked individual beam 14, which connects a central object field pointto a point in the pupils of the projection optics 6 in the pupil planes22, 24, will be referred to hereinafter also as the principal ray 25 ofthe central field point. From reflection on the sixth mirror 21, theprincipal ray 25 of the central field point encloses with the imageplane 8 roughly a right angle, i.e. extends also substantially parallelto the z-axis of the projection exposure system 1. This angle is in anycase greater than 85°. From reflection on the sixth mirror 20, theprincipal ray 25 extends along an optical axis 26 of a refractivesubunit 27, connected to the reflective subunit 15, of the projectionoptics 6. The intersection of the optical axis 26 of the refractivesubunit 27 with the image plane 8 is located centrally in the imagefield.

The refractive subunit 27 has a total of six lenses 28 to 33 which arenumbered in the order of the beam path between the object plane 4 andthe image plane 8.

A second intermediate image plane 34 of the projection optics 6 islocated between the sixth mirror 21 of the reflective subunit 15 and thefirst lens 28 of the refractive subunit 27. The numerical aperture ofthe projection optics 6 in the intermediate image plane 34 is 0.37.

Located between the first lens 28 and the second lens 29 is a thirdpupil plane 35 of the projection optics 6, in which for example anaperture stop can be arranged.

The image field of the projection optics 6 in the image plane 8 isrectangular. Parallel to the x-direction, the image field has anextension of 26 mm. Parallel to the y-direction, the image field has anextension of 6 mm. The optical axis 26 passes through the image fieldcentrically, i.e. at the intersection of the diagonals thereof.

The projection optics 6 dispenses with a beam splitter cube and a planarfolding mirror and thus has a particularly small number of opticalcomponents.

FIG. 3 shows the field profile of the wave front of the projectionoptics 6 according to FIG. 2 in the image field. The scale of the y-axisis in this case shown stretched compared to that of the x-axis. FIG. 3shows a wave front correction to a value of at most 80 mλ. The smallestwave front error occurs at relatively high positive y-values, based onthe x-axis, centrically and at relatively high negative y-valueseccentrically.

FIG. 4 shows the profile of the distortion via the image field of theprojection optics 6 according to FIG. 2. The scale of the x-axis and they-axis corresponds to the scale of FIG. 3. The distortion is correctedto a maximum value of approximately 25 nm. This maximum value occurs athigh y-values located, with respect to x, at the edge of the imagefield. As may be seen, the image error profile no longer extendsrotationally symmetrical-ly around the centre of the field, as is thecase in conventional rotationally symmetrical systems having a centredobject field and image field.

The generation of a free-form surface 36 from a rotationally symmetricalreference surface 37 will be described hereinafter with reference toFIG. 5.

Firstly, information for characterising the viewed free-form surface isobtained with the aid of an optical design program. The referencesurface 37 may, for example, be a rotationally symmetrical asphere. Thedesign information may include the radius of curvature of the referencesurface 28, which is also denoted as 1/c, wherein c denotes the peakcurvature of the reference surface 37. The information additionallyincludes a conical constant k of the reference surface 37 and polynomialcoefficients describing the reference surface 37.

Alternatively or additionally, information characterising the referencesurface 37 can be obtained from a surface measurement of a referencemirror surface, for example using an interferometer. A surfacemeasurement of this type produces a function z′(x′, y′) describing thereference surface 37, wherein z′ denotes the pitch of the referencesurface 37 along the z′-axis for various (x′, y′) coordinates, as shownin FIG. 5.

This first step in the design of the free-form surface 36 additionallyincludes determining that portion of the initially unlimited mirrorsurface, defined only by the surface description, that is actuallyutilised for the reflection of illumination or imaging light 3 duringthe mapping of the object field into the image field. This region isalso referred to as a footprint. The footprint of the mirror can bedetermined, at least approximately, by ray tracing of the projectionoptics 6. Examples of a possible footprint in the x-dimension areindicated in FIG. 5. xmin denotes the lower limit and xmax the upperlimit for the exemplary footprint. The data above xmax and below xmin isalso calculated within certain limits so as to avoid undesirable edgeeffects when determining the free-form surface 36. Once the informationcharacterising the reference surface 37 has been determined, a localcoordinate system for the reference surface 37 is introduced, in whichboth the decentring and the tilting of the reference surface 37 are eachzero. The z′-axis is therefore the rotational axis of symmetry of theaspherical reference surface 37 or else, if the reference surface wasobtained by surface measurement, the optical axis of the measuringdevice, for example the interferometer. The z′-axis is generallydisplaced and tilted parallel relative to the z-axis of thexyz-coordinate system of the projection exposure system 1. This appliesaccordingly to the other coordinate axes x′, y′. This paralleldisplacement or tilting is defined in the initial step of the opticaldesign of the free-form surface.

As an alternative to an asphere, the reference surface 37 may also be aspherical surface. The coordinate origin x_(c), y_(c), z_(c) fordescribing the spherical reference surface 37 generally differs from theorigin of the xyz-coordinate system of the projection exposure system 1.

Once the reference surface 37 has been determined, a local distance d,(i=1 . . . N) between a number of points on the reference surface 37 andpoints on the freeform surface 36 is determined parallel to the z′-axis.The various local distances d_(i) are then varied until a group ofsecondary conditions is fulfilled. These secondary conditions arepredetermined limit values for specific imaging errors and/orillumination properties of the projection optics 6.

The free-form surface can be described mathematically by the followingequation:

$Z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {\sum\limits_{j = 2}^{66}\; {C_{j}X^{m}Y^{n}}}}$wherein: $j = {\frac{\left( {m + n} \right)^{2} + m + {3n}}{2} + 1}$

Z is the pitch of the free-form surface parallel to a Z-axis which may,for example, be parallel to the z′-axis according to FIG. 5.

c is a constant corresponding to the peak curvature of a correspondingasphere. k or K corresponds to a conical constant of a correspondingasphere. C_(j) are the coefficients of the monomials X^(m)Y^(n).Typically, the values of c, k and C_(j) are determined on the basis ofthe desired optical properties of the mirror within the projectionoptics 6. The order of the monomial, m+n, can be varied as desired. Arelatively high-order monomial can lead to a design of the projectionoptics 6 having better image error correction but is more complex tocalculate. m+n may assume values between 3 and greater than 20.

Free-form surfaces can also be described mathematically by Zernikepolynomials, an account of which is provided, for example, in the manualof the optical design program CODE V®. Alternatively, free-form surfacescan be described with the aid of twodimensional spline surfaces.Examples of these include Bezier curves or non-uniform rational basissplines (NURBS). Two-dimensional spline surfaces can, for example, bedescribed by a network of points in an xy-plane and associated z-valuesor by these points and gradients associated therewith. Depending on therespective type of spline surface, the whole surface is obtained byinterpolation between the network points using, for example, polynomialsor functions having specific properties in terms of their continuity anddifferentiability. Examples of these include analytic functions.

FIG. 6 is, again, a slightly modified view of the projection exposuresystem 1 to illustrate a further value characteristic of the projectionoptics 6, i.e. the object/image offset d_(ois). The object/image offsetis defined as the distance between a perpendicular projection of thecentral object point onto the image plane 8 and the central image point.Equivalent to this definition is a definition of the object/image offsetas the distance between the central object point and the intersection ofthe optical axis 26 of the refractive subunit with the object plane.This equivalence is due to the fact that the central image point islocated on the optical axis of the refractive subunit. In the projectionoptics 6 according to FIG. 2, the object/image offset d_(ois) is 0. Inparticular, the object field is in the projection optics 6 at a distanceof less than 50 mm from the intersection of the optical axis 26 of therefractive subunit 27 with the object plane 4. This intersection islocated centrically in the object field.

Characteristic parameters of the projection optics will be recapitulatedhereinafter. The wavelength of the illumination light 3 is 193.3 nm at abandwidth of 0.3 pm. The image-side numerical aperture of the projectionoptics is 0.8. The image field size is 6×26 mm². The projection opticsis telecentric on the input and output sides.

The following first table sets out the radii R, i.e. the reciprocalvalues of the peak curvatures c, and the distances between the opticalcomponents of the projection optics 6 in the z-direction (thickness).Mirror 1 to mirror 6 denote in this case the mirrors M1 to M6 of theprojection optics 6. Lens 1 a and lens 1 b denote the object-side andthe image-side surface of the lens 28. Accordingly, the lines from lens2 a denote the subsequent lens surfaces up to the image-side surface ofthe lens 33.

The subsequent second, two-line table sets out the index of refractionof the material SILUV, selected for the lenses 28 to 33, within thebandwidth of the illumination light.

Surface Radius Thickness Mode/Material Object INFINITY 270.081 Mirror 1−252.965 −238.081 REFL Mirror 2 −406.220 891.920 REFL Mirror 3 −979.012−891.920 REFL Mirror 4 1445.893 891.920 REFL Mirror 5 223.730 −194.805REFL Mirror 6 214.750 447.659 REFL Lens 1a 917.582 80.000 SILUV Lens 1b−395.946 113.054 STOP INFINITY 61.038 Lens 2a 728.174 80.000 SILUV Lens2b −447.709 72.468 Lens 3a 1274.086 80.000 SILUV Lens 3b −344.145 16.037Lens 4a 287.238 83.816 SILUV Lens 4b −560.707 2.069 Lens 5a 291.34159.824 SILUV Lens 5b 584.886 15.839 Lens 6a 471.437 55.444 SILUV Lens 6bINFINITY 10.001 Image INFINITY 0.000

Wavelength 193.400 193.300 193200 SILUV 1.560332 1.560491 1.560650

The following table sets out the coefficients C_(j) of the monomialsX^(m)Y^(n) in the above-indicated free-form surface equation for themirrors M1 to M6.

Coefficient M1 M2 M3 M4 M5 M6 K −7.021612E−01 −2.157890E+00−7.667374E−01 −1.806988E+00 1.063503E+00 −1.746422E−01 Y 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X29.095147E−04 3.668196E−04 2.973173E−04 −4.649230E−05 −1.183376E−03−3.419562E−04 Y2 6.389416E−04 2.560392E−04 2.978561E−04 −7.671144E−05−9.182562E−04 −3.007775E−04 X2 −8.076564E−07 −6.252867E−07 −3.355736E−08−6.400021E−08 9.454888E−06 2.940059E−07 Y3 −2.117970E−06 −5.447745E−07−2.754244E−08 −1.460071E−08 6.613284E−07 1.847585E−07 X4 −2.138970E−10−8.640102E−09 3.920489E−11 8.751552E−11 −4.472091E−08 −1.426177E−09 X2Y2−6.007701E−09 −2.165820E−08 6.203578E−11 3.851618E−11 1.500646E−07−1.613361E−09 Y4 −9.250730E−09 −6.779145E−09 6.568625E−11 1.229035E−10−1.127945E−07 −1.408353E−09 X4Y −2.056552E−11 −8.895852E−12−1.220037E−14 2.660699E−13 −2.255937E−09 4.795621E−12 X2Y3 −4.739414E−115.099086E−12 −6.214499E−14 −8.442145E−15 3.342074E−09 7.024093E−12 Y5−3.327059E−11 2.955670E−11 7.082062E−14 8.855762E−14 −1.023558E−09−2.144019E−13 X6 −1.832168E−14 −1.470440E−13 −5.507976E−18 −3.219684E−17−1.402903E−11 −2.945400E−15 X4Y2 −1.922635E−13 −1.214485E−129.317572E−17 5.849016E−16 −7.112824E−11 −4.171618E−15 X2Y4 −1.575366E−13−3.577373E−14 2.207096E−16 −7.153565E−17 3.488042E−11 −2.806538E−14 Y6−8.968203E−14 7.845522E−13 7.803209E−17 2.198865E−17 3.795039E−12−3.038365E−14 X6Y −1.281116E−16 −2.627145E−15 −4.849826E−20−9.242214E−20 −4.829253E−13 1.984066E−16 X4Y3 −9.140726E−16−3.557075E−15 3.159899E−19 6.234055E−19 −1.008249E−12 4.813599E−16 X2Y5−2.413169E−16 1.757810E−15 9.795658E−19 −2.688674E−19 1.917495E−133.980208E−16 Y7 −3.754943E−16 5.872039E−15 −9.221194E−20 −7.819713E−211.901940E−13 3.896110E−17 X8 5.695077E−20 7.943905E−18 −5.452381E−23−1.225852E−22 3.587655E−15 −4.740965E−19 X6Y2 −1.254319E−18 2.689048E−16−1.335406E−22 −3.678195E−23 −7.156201E−15 −7.811848E−19 X4Y4−1.658577E−18 3.240175E−16 4.953597E−22 4.109732E−22 −6.930341E−156.890495E−19 X2Y6 4.989886E−20 1.087723E−17 1.704149E−21 −2.812559E−22−3.664911E−16 2.629890E−19 Y8 −1.551184E−18 2.695661E−17 −3.595780E−222.753738E−23 1.738454E−15 5.135593E−20 X8Y −9.973638E−21 1.930861E−18−1.667244E−25 −2.536190E−25 9.857871E−17 −1.714248E−21 X6Y3−7.606565E−21 6.217397E−18 −2.273189E−25 1.022753E−25 −6.200635E−17−6.960381E−21 X4Y5 4.843747E−22 4.566777E−18 2.515461E−25 2.188836E−25−1.792880E−17 −9.623084E−21 X2Y7 1.828007E−23 −1.014580E−19 1.345882E−24−1.041776E−25 −9.929708E−18 −5.083648E−21 Y9 −3.672869E−21 3.548978E−20−3.911346E−25 3.732796E−26 6.950217E−18 1.389683E−21 X10 −2.139041E−233.593455E−21 −1.343515E−29 −1.288642E−29 3.181161E−19 1.291645E−23 X8Y2−4.164394E−23 1.984011E−20 −1.655578E−28 −1.291875E−28 4.620193E−198.021568E−23 X6Y4 −1.391966E−23 3.356495E−20 −1.667437E−28 9.320319E−29−2.306996E−19 1.145367E−22 X4Y6 2.959150E−24 1.871129E−20 4.168020E−308.701089E−29 −9.628058E−21 −2.791674E−23 X2Y8 −1.066355E−24 2.488743E−224.086345E−28 3.310765E−31 −4.149211E−20 −9.425279E−23 Y10 −3.494516E−241.039002E−22 −1.471588E−28 1.304435E−29 9.112470E−21 −4.264041E−23Nradius 1.000000E+00 1.000000E+00 1.000000E+00 1.000000E+00 1.000000E+001.0000000E+00

The following two-line table sets out for the mirrors M1 to M6 theamounts in mm by which each mirror, starting from an initial design, isdecentred (Y-decentre) and rotated (X-rotation). This corresponds to theparallel displacement and the tilting during the above-describedfree-form surface design process. Displacement is in this case carriedout in the y-direction and tilting about the x-axis.

Coefficient M1 M2 M3 M4 M5 M6 Y-decentre 163.446 −22.380 −119.285124.459 36.347 −37.190 X-rotation 29.034 −0.370 3.988 1.234 7.717 3.171

The following table sets out the aspherical constants for the curvedsurfaces of the lenses 28 to 33.

Lens 1b Lens 2a Lens 3° Lens 4a Lens 5b Lens 6a K −5.320892E+000.000000E+00 0.000000E+00 −4.766092E+00 0.000000E+00 0.000000E+00 A0.000000E+00 −1.988938E−08 −1.463509E−09 0.000000E+00 −9.475075E−08−3.645569E−08 B 1.887087E−13 −1.531838E−13 −6.363694E−14 −6.652848E−137.801991E−12 9.565989E−12 C −2.425975E−18 −1.077221E−18 4.281797E−192.251669E−17 −3.331273E−16 −9.233817E−16 D 1.942174E−24 −2.420592E−23−1.251778E−23 −6.275180E−23 1.114370E−20 7.314484E−20 E 1.934240E−27−8.586290E−28 3.666866E−29 −3.691459E−27 −5.980525E−25 −9.261069E−24

K and A to E are in this case coefficients in the following asphericalequation:

$Z = {\frac{{ch}^{2}}{\sqrt{1 - {\left( {1 + K} \right)c^{2}h^{2}}}} + {Ah}^{4} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10} + {Eh}^{12}}$

In this case, Z is the pitch of the aspherical surface, c is the peakcurvature, K is the conicity, h denotes the respective location on thelense surface at which the pitch is calculated (h²=x²+y²). Thecoefficients A to E are assigned to the respective even-numbered ordersof h.

FIG. 7 shows a projection optics 38 which can be used instead of theprojection optics 6 in the projection exposure system 1. Components orreference variables corresponding to those previously describedhereinbefore with reference to FIGS. 1 to 6 have the same referencenumerals and will not be discussed again in detail. The imaging beampath through the projection optics 38 is shown in FIG. 7 merely based onthe principal ray 25 of the central field point.

Starting from the projection plane 4, the projection optics 38 hasfirstly a refractive subunit 39. The refractive subunit is arranged insuch a way that the principal ray 25 extends through the refractivesubunit 39 along an optical axis 40 of the refractive subunit 39. Therefractive subunit 39 can include one or more lenses.

After the refractive subunit 39, the illumination light 3 is reflectedby a reflective subunit 41 of the projection optics 38. The reflectivesubunit 41 has two mirrors 42, 43, both of which have static reflectionfree-form surfaces which cannot be described by a rotationallysymmetrical function. The mirrors 42, 43 are numbered in accordance withtheir order in the imaging beam path in the projection optics 38. Themirrors 42 and 43 are concave.

After the mirror 43, the illumination light 3 extends through a secondrefractive subunit 44 of the projection optics 38. The principal ray 25extends, in turn, along an optical axis 45 of the second refractivesubunit 44. The second refractive subunit 44 can have one lens or aplurality of lenses.

The projection optics 38 has between the object plane 4 and the imageplane 8 one or more intermediate image planes. The intermediate imageplanes can, for example, be arranged between the first refractivesubunit 39 and the reflective subunit 41 or else between the reflectivesubunit 41 and the second refractive subunit 44.

FIG. 8 is an illustration similar to FIG. 7 of a projection optics 46which can be used instead of the projection optics 6 in the projectionexposure system 1. Components or reference variables corresponding tothose previously described hereinbefore with reference to FIGS. 1 to 7have the same reference numerals and will not be discussed again indetail.

The illumination light 3 in the projection optics 46 is illustrated inFIG. 8 based on three individual beams 14 emanating from an objectpoint. The central individual beam 14 is in this case the principal ray25.

Starting from the object field 4, the illumination light 3 first passesthrough a refractive subunit 47 which can be configured so as tocorrespond to the refractive subunit 39 according to FIG. 7. Theprincipal ray 25 passes through the refractive subunit 47, on theoptical axis 48 thereof.

After the refractive subunit 47, the illumination light 3 passes througha reflective subunit 49 of the projection optics 46. The reflectivesubunit 49 has a total of four reflecting mirrors 50, 51, 52, 53 whichare numbered in the order in which they are acted on in the imaging beampath. The mirrors 50 to 53 are concave. However, convex mirrors can alsobe used in this subunit. A pupil plane of the projection optics 46 isarranged in the region of the second mirror 51. An intermediate imageplane 54 of the projection optics 46 is arranged between the mirrors 51and 52. The optical axis 48 is, for example, perpendicular to theintermediate image plane 54. Between the mirrors 51 and 52, theprincipal ray 25 extends substantially parallel to the optical axis 48.The principal ray 25 can also extend between the mirrors 51 and 52 at anangle to the optical axis 48.

After the last mirror 53 of the reflective subunit 49, the illuminationlight 3 passes through a second refractive subunit 55 of the projectionoptics 46.

In relation to the intermediate image plane 54, the optical componentsof the three subunits 47, 49 and 55 of the projection optics 46 arearranged substantially symmetrically to one another. The optical axis ofthe second refractive subunit 55 coincides with the optical axis 48 ofthe first refractive subunit 47. A further pupil plane of the projectionoptics 46 is arranged in the region of the third mirror 52.

The four mirrors 50 to 53 of the reflective subunit 49 all have areflection free-form surface which cannot be described by a rotationallysymmetrical function. The free-form surfaces of the mirrors 50, 52 and53 are in this case static. The reflection surface of the second mirror51 is deformable, i.e. non-static, in its configuration. Alternativelyor additionally, the reflection surface of the third mirror 52 can alsobe deformable in its configuration. This allows, for example, drifteffects, which can occur during projection exposure with the projectionexposure system 1, to be corrected. A correction mechanism includingcorrections sensors can be used for this purpose, as described in DE 10120 446 C1. The deformable free-form mirror 51 can be formed from a largenumber of micromirror segments which can be tilted individually viaactuators associated therewith. Micromirror arrays of this type areknown to a person skilled in the art. The tilting is activated by thecorrection mechanism in accordance with defined values calculated fromthe values determined by the correction sensors. There is sufficientspace for the actuators of the micromirror array after the mirror 51, asno beam path extends at this location. Alternatively, a deformablefree-form mirror can be configured as a monolithic mirror, there beingattached to the back of the deformable free-form mirror actuators whichare able to deform the mirror in the manner of a membrane.

Instead of the deformable free-form mirror 51, a static free-form mirrorcan also be used.

The projection optics 46 has an object/image offset of 0.

To manufacture a microstructured structural part with the aid of theprojection exposure system 1, the reticle 9 and the wafer 10 are firstprovided. Subsequently, a structure on the reticle 9 is projected onto alight-sensitive layer of the wafer 10. As a result of this and bysubsequent machining, a microstructure is produced on the wafer 10.

Other embodiments are in the claims.

1. A projection optics configured to image an object field in an objectplane into an image field in an image plane, the projection opticscomprising: a curved mirror; and a refractive subunit, wherein: theprojection optics has an object/image offset which is less than 50 mm;an individual beam of an imaging beam path is a principal ray whichconnects a central object field point to a centrally illuminated pointin a pupil of the projection optics in a pupil plane of the projectionoptics; and the projection optics is configured to be used inmicrolithography.
 2. The projection optics according to claim 1, whereinthe image plane is arranged parallel to the object plane.
 3. Theprojection optics according to claim 1, wherein the object/image offsetis zero.
 4. The projection optics according to claim 1, wherein anintersection of an optical axis of the refractive subunit with theobject plane is located in the object field.
 5. The projection opticsaccording to claim 4, wherein the intersection of the optical axis ofthe refractive subunit with the object plane is centred in the objectfield.
 6. The projection optics according to claim 1, wherein anintersection of the optical axis of the refractive subunit with theimage plane is located in the image field.
 7. The projection opticsaccording to claim 6, wherein the intersection of the optical axis ofthe refractive subunit with the image plane is centred in the imagefield.
 8. The projection optics according to claim 1, wherein theprojection optics comprises six mirrors.
 9. The projection opticsaccording to claim 1, wherein the refractive subunit has at most eightlenses.
 10. The projection optics according to claim 1, comprising animage-side refractive subunit and an object-side refractive subunit,wherein there are at least two mirrors between the image-side refractivesubunit and the object-side refractive subunit, at least one of the atleast two mirrors has a free-form surface.
 11. The projection opticsaccording to claim 10, wherein an intersection of the optical axis ofthe object-side refractive subunit with the object plane is located inthe object field, and an intersection of the optical axis of theimage-side refractive subunit with the image plane is located in theimage field.
 12. The projection optics according to claim 11, whereinthe intersection of the optical axis of the object-side refractivesubunit with the object plane is centred in the object field, and theintersection of the optical axis of the image-side refractive subunitwith the image plane is centred in the image field.
 13. The projectionoptics according to claim 10, wherein a principal ray of a central fieldpoint between two mirrors arranged between the object-side andimage-side refractive subunits extends parallel to the optical axis ofthe object-side and image-side refractive subunits and set apart fromthis optical axis.
 14. The projection optics according to claim 10,wherein the optical axes of the object-side and image-side refractivesubunits extend parallel and set apart from each other.
 15. Theprojection optics according to claim 1, further comprising at least onemirror which is deformable in the region of a pupil plane of theprojection optics.
 16. The projection optics according to claim 1,wherein the projection optics is configured to illuminate an image fieldwhich is larger than 1 mm².
 17. The projection optics according to claim1, wherein the projection optics has an image-side numerical aperture ofat least 0.5.
 18. The projection optics according to claim 1, whereinthe projection optics is telecentric on at least one of the object andimage sides.
 19. The projection optics according to claim 1, furthercomprising a light source configured to generate illumination lighthaving a wavelength in the range of from 126 to 248 nm.
 20. Theprojection optics according to claim 1, wherein the projection opticshas at least one intermediate image plane in the imaging beam pathbetween the object plane and the image plane.
 21. The projection opticsaccording to claim 20, wherein the refractive subunit is arrangedbetween the intermediate image plane and the image plane.
 22. Aprojection exposure system, comprising: an illumination opticsconfigured to guide illumination light toward an object field in anobject plane; and a projection optics according to claim 1, wherein theprojection exposure system is a microlithography projection exposuresystem.
 23. A method, comprising: Using the projection exposure systemof claim 22 to produce a microstructured element.
 24. The method ofclaim 23, further comprising: projecting a structure of a reticle onto alight-sensitive layer of a wafer via the projection exposure system.